Tanning is one process stage in manufacturing animal skins into durable leather. In tanning the protein structure of the skin is permanently altered. The tanning process aims at, in addition to avoiding rottening of the skin, increasing resistance to water, humidity and usage together with increasing flexibility, anti-allergenic properties and visual attractiveness. Pretreatment processes are required before tanning can take place such as splitting, deliming and/or pretanning processes like bating, decreasing, frizing and bleaching which are typically included in the processing stages.
There are three dominating tanning methods; aldehyde or synthan tanning, mineral tanning predominated by chrome tanning and vegetable tanning. Each of these tanning agents produces leathers with different properties. However, increasingly environmentally friendly solutions such as chrome or aldahyde free tanning agents are favoured, especially within e.g. automotive industry.
Chrome tanning with basic chrome sulphate is used in 85% of the world's tanned leather processing. A major advantage in this approach is the very high shrinkage temperature, 100° C. or more, provided to the finished leather by the method. The major future drawback will be the environmental problems related to the use of chromium and depletion of the availability of the ore. The visual appearance of bluish hue in colour is another unwanted product feature. In chrome tanning the chromium salts crosslink collagen protein molecules which make the hides less susceptible to effects of heat and putrefaction. The chrome tanning process, however, requires use of additional chemicals such as buffering and basification solutions. Prior to the introduction of the basic chromium, several steps are required to produce a tannable hide including scudding, liming, introduction of alkali agents such as sodium hydroxide, deliming, restoring neutral pH, bating, or softening the skin with enzymes, pickling i.e. lowering pH of the hide with salt and sulphuric acid. The pH is very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit in between the fibres and residues of the collagen. Once the desired level of penetration of chrome into the hide is achieved, pH of the material is raised again i.e.“basified” to facilitate the process. At this stage, the chrome tanned skins obtain the bluish colour.
Modern chrome-free mineral tanning comprises the use of sodium aluminium silicates (NAS) providing tanned leather with whitish colour hue. Synthetic zeolites have been tested also providing durable, resistant, readily machine processable, shavable and dimensionally stable leather products. A typical drawback in these processes is the lowered shrinkage temperature, TS, of the hides compared to chrome tanning due to formation of less stable complexes with collagen.
Vegetable tanning is an earlier process to mineral tanning the name originating from the use of tannin in the process. Tannins bind to the collagen proteins in the hide and coat them causing them to become less water-soluble, more resistant to bacterial attack, and increasing the hide flexible. This tanning method is, however, quite slow and has been largely overcome by the more efficient chrome tanning which is faster, taking less than a day, and produces a stretchable leather which is excellent for use e.g. in handbags and garments. Vegetable tanning is still in use for e.g. furniture and luggage leathers.
In aldehyde tanning amino groups of collagen are reacted with aldehydes. The shrinkage temperature obtained is adequate, about 75° C., but the colour hue of the tanned hide is yellowish, or sometimes even orange. The major drawback is that the hide can only partly be modified. Aldehyde tanning is typically used in conjunction with other tanning agents but it is not suitable as the sole tanning agent. The possible formaldehyde release is another concern. Specifically, this is an issue in the automotive and toy industry wherein strict concentration limits have been imposed.
Costantini et al., “Studies on the tanning reactions of zeolite” in JALCA, vol. 95, 2000, pp. 125-137 discloses a study on the reactions involved in pretanning or tanning when using zeolite based masking agents. The hydrothermal stability of sodium aluminum silicate is considered to be too low for use in tanning solely by a zeolite. The role of pH and acidic solutions in aluminosilicate breakdown are emphasized and discussed in detail. Maleic acid and phthalic acid are considered the only possible carboxylic acids to elevate the shrinkage temperature to an acceptable level. The shrinkage temperatures are determined by differential scanning spectroscopy. The hides are pretanned before the actual tanning.
One pre-tannage system for leather includes treating the hide with a zeolite material, such as sodium aluminium silicate in a first pre-tannage step and thereafter treating the hide with one or more modified aldehyde tanning agents. At this stage, the hide is suitable for a number of different tanning steps namely chrome tannage, vegetable tannage, synthetic tannage or combinations thereof.
Sodium aluminium silicate used for tanning leather must be added in the acidic phase with the result that it hydrolyses to alkaline aluminium salts and polysilicic acids. As the sodium aluminium silicate has not enough time to fully penetrate into the skin and become an active tanning agent prior to the decomposition, the tanning action will be restricted to the outer layers of the hide.
Others have disclosed a process of tanning for the production of dressed fur skins. In this process pickled fur skins are subjected to the action of an aqueous liquor containing tanning agents. A water-insoluble aluminosilicate containing bound water, of the formula (Cat2/nO)x.Al2O3.(SiO2)y wherein Cat represents a cation selected from the group consisting of alkali metals, bivalent metal ions, trivalent metal ions and mixtures thereof; n represents an integer from 1 to 3; or x is a number of from 0.5 to 1.8; and y is a number of from 0.8 to 50, is added to the pickling bath as the tanning agent. Auxiliary tanning agents such as chrome and further chemicals such as carboxylic acids having at least two carboxyl groups may be added into the pretanning stage and tanning.